1. Field of the Invention
The present invention telales, in general, to lithography useful for fabricating semiconductor devices and, more particularly, to a method for the coating of photoresist, capable of allowing the photoresist to be uniformly thick regardless of the topology of the patterns formed thereunder. Also, the present invention is concerned with an apparatus therefor.
2. Description of the Prior Art
Photoresist, typically used as a photomask, if coated on, for example, a wafer by a spin coat process, has a smooth surface irrespective of the topology of the wafer, that is, patterns formed on the wafer. Thus, the resulting photoresist is different in its thickness in accordance with the position of the patterns.
In order to better understand the background of the invention, reference is made to FIG. 1 wherein photoresist coated on a wafer by a prior art technique is shown. The photoresis 3 is spin-coated on a wafer 1 which has been provided with a variety of patterns 2, showing a smooth surface. As seen in this figure, the thickness t.sub.1 of the photoresist 3 from the surface to the patterns is different from that of t.sub.3 from the surface to the wafer 1. As the topology of the patterns is more uneven, the difference between the thicknesses t.sub.1 and t.sub.2 becomes larger.
Due to the different thicknesses in the photoresist, it is difficult to set the energy of a light illuminated thereupon. For example, if the energy of light is determined on the basis of the thickness t.sub.1 from the smooth surface to the wafer 1, the photoresist 3 coated over the patterns 2 is exposed to too intensive light. This results in a narrower photoresist pattern than is predetermined after development process. On the other hand, if the energy of light is based on the thinner thickness t.sub.2 from the smooth surface to the patterns 2 provided on the wafer 1, the photoresist over the wafer does not absorb enough light to form a pattern. Hence, there is a photosensitive film as the area wherein no photosensitive film must remain after the development process.
As a result, the prior technique has difficulty in forming fine patterns and thus, causes a problem that the process margin in fabricating semiconductor devices is reduced, so as to lower production yield.